BoilerFaultCodesViessmann 18 Fault Code
The Viessmann 18 fault code means: your boiler has lost its connection to the outside temperature sensor, meaning it cannot automatically adjust your heating based on the weather.. It appears on 12 Viessmann models in our database.
Technical description: Controls according to 0°C outside temperature: Outside temperature sensor lead break
What causes the Viessmann 18 fault?
This fault occurs when the boiler's control board loses the electrical signal from the sensor located on the outside of your home. The most common reason is a broken or corroded wire between the sensor and the boiler, often caused by exposure to the elements or accidental damage. In some cases, the internal components of the sensor itself may have failed due to age.
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
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Viessmann models that show the 18 fault code
Click a model for the full fault code list and model-specific repair notes.
- Vitodens 050-W Repair: £90-180
- Vitodens 050-W BPJD Repair: £120-200
- Vitodens 100-W Repair: £120-200
- Vitodens 100-W WB1A Repair: £120-200
- Vitodens 100-W WB1B Repair: £120-200
- Vitodens 111-W Repair: £120-200
- Vitodens 200-W Repair: £90-180
- Vitodens 222-F Repair: £90-180
- Vitodens 242-F B2UA Repair: £90-180
- Vitodens 242-F FB2B Repair: £90-180
- Vitodens 300-W Repair: £90-180
- Vitodens 333-F Repair: £90-180
Source
Viessmann 18 fault code information on this page is sourced and cross-checked against the official Viessmann service / installation manuals for each affected model.
- Viessmann Vitodens 050-W BPJD manual (PDF)
- Viessmann Vitodens 100-W manual (PDF)
- Viessmann Vitodens 100-W WB1A manual (PDF)
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 7 July 2026